The present invention relates to an electrolyte as well as a method for the electroless deposition of metals, particularly layers of nickel, copper, cobalt, boron or gold, as well as layers of alloys comprising at least one of the aforementioned metals as alloying metal. Furthermore, the present invention relates to the use of β-amino acids as stabilizers in electrolytes for the electroless deposition.
The present invention further relates to an organic stabilizer for electroless plating processes, and an electrolyte for the electroless deposition of a metal layer on a substrate, comprising a metal ion source for the metal to be deposited, a reducing agent, a complexing agent, a stabilizer and preferably an accelerator, as well as a method for the electroless deposition of a metal layer on a surface from an electrolyte according to the invention. Further the invention relates to a use of at least one carboxylic acid and/or at least one salt of a carboxylic acid.
Among electrolytic methods for the plating of substrates with metal layers, electroless plating methods have long been known from the state of the art. By electroless plating, also known as chemical plating, the coating of almost every metal and a huge number of non-conductive substrate surfaces is possible. The electroless deposited metal layers differ from the galvanically deposited metal layers, i. e. those layers deposited by the use of an external current, in physical as well as mechanical aspects. Often, metal alloy layers with non-metal elements, like for example cobalt/phosphor, nickel/phosphor or boron carbide layers are deposited by means of electroless deposition methods. In this respect, electroless deposited layers in many cases differ also in their chemical nature from the galvanically deposited layers.
One major advantage of the electroless deposited metal layer is the outline accuracy of the layer thickness of the deposited layer independent from the substrate geometry, which feature makes the electroless methods the first choice in the area of printed circuit boards (PCB) manufacturing, and here especially for the metallization of through hole contacts, vias and trenches.
Many times, electroless methods are also used for the coating of other non-conductive substrates, like for example plastic substrates, to render the surface of such substrates conductive and/or to change the appearance of the substrate in aesthetic respect. Furthermore, by the deposited layers, the material properties of the coated substrate can be improved or amended. Especially, the corrosion resistance or the hardness of the surface and/or the wear resistance of the substrate can be improved.
Electroless plating methods are based on an autocatalytic process, in which process the metal ions comprised in the electrolytes are reduced to the elemental metal by a reducing agent which is oxidized during this redox reaction.
A reducing agent commonly used in the field of electroless deposition of metals on substrate surfaces is sodium hypophosphite. However, also other reducing agents are used in dependency of the metals to be deposited.
U.S. Pat. No. 6,146,702 discloses an electroless nickel cobalt phosphorus composition and plating process. The process is provided for enhancing the wear resistance of aluminium and other materials by depositing on the substrate a nickel, cobalt, phosphorus alloy coating using an electroless plating bath to provide a plated alloy having a cobalt content of at least about 20% by weight and a % Co/% P weight ratio of at least about 5.
European patent application EP 1 413 646 A2 discloses, for example, an electrolyte for the electroless deposition of nickel layers having internal compressive stress. The electrolyte disclosed in this application comprises a metal salt of the metal to be deposited, a reducing agent, a complexing agent, an accelerator, and a stabilizer.
Here, the accelerator is used to increase the deposition rate of the metal on the substrate surface.
In known electroplating baths it is necessary to use a stabilizer to avoid the uncontrolled plateout (wild deposition) of the electrolyte, which means the unregulated wild deposition of metal on the substrate surface. Hitherto, in the state of the art, heavy metals like lead, bismuth, zinc or tin are used as stabilizers. According to common environmental regulations [ROHS (Restriction of the use of certain hazardous substances), WEEE (Waste electrical and electronic equipment), ELV (End of lifetime of vehicles)] prior to the disposal of expended electrolytes and the co-deposition of heavy metals such heavy metals have to be withdrawn from the aqueous solution used as electrolyte in an adequate treatment step. Also when the heavy metals are comprised in the electrolyte only in small amounts, such a treatment causes additional expenses for the disposal. Therefore, the use of heavy metal in electrolytes for the deposition of metal layers has to be avoided. In some other types of electrolytes, like for example electrolytes for the electroless deposition of copper, cyanides are used as stabilizers. Like heavy metal ions, such cyanides are subject to environmental regulations. The same is true for selenium compounds which are also commonly used as stabilizers.
Furthermore, there is an interest in the art of metal plating to avoid sulfur compounds which comprise sulfur in an oxidation state between −2 and +5, since these compounds are also subject to environmental regulations. However, by now such compounds are often needed in the electrolytes to gain good plating results.